Nitride semiconductors containing nitrogen (N) as a Group V element are regarded as promising materials for short-wave light emitting elements because of their band gap sizes. Gallium nitride-based compound semiconductors (GaN-based semiconductors: AlxGayInzN, 0≦x, y, z≦1, x+y+z=1), in particular, are being actively studied, and have been put into practical use in the form of blue light-emitting diodes (LEDs), green LEDs, and semiconductor lasers that use the GaN-based semiconductor as a material.
The GaN-based semiconductors each have a wurtzite crystal structure. FIG. 1 schematically illustrates the unit cell of GaN. In crystal of an AlxGayInzN (0≦x, y, z≦1, x+y+z=1) semiconductor, some of Ga atoms illustrated in FIG. 1 can be substituted for Al and/or In.
FIG. 2 illustrates four fundamental vectors, a1, a2, a3, and c which are commonly used to express planes of a wurtzite crystal structure in four-digit indices (Hexagonal Miller-Bravais indices). The fundamental vector c runs in a [0001] direction, which is called a “c-axis”. A plane perpendicular to the c-axis is called a “c-plane” or a “(0001) plane”. The “c-axis” and the “c-plane” are sometimes written as “C-axis” and “C-plane”, respectively. The capital-letter notation is used in the accompanying drawings for easier viewing.
When manufacturing a semiconductor element by using a GaN-based semiconductor, a c-plane substrate, i.e., a substrate having a (0001) plane as a surface is used as a substrate on which a GaN-based semiconductor crystal is grown. However, electrical polarization is formed on the c-plane where the positions of a Ga atomic layer and a nitrogen atomic layer are slightly misaligned with each other in a c-axis direction. The “c-plane” is therefore also called as a “polar plane”. As a result of the electrical polarization, a piezo-electric field is generated along the c-axis direction in a quantum well of InGaN in an active layer. The generation of the piezo-electric field in the active layer shifts the distribution of electrons and holes within the active layer through the quantum-confined Stark effect of carriers, thereby lowering the internal quantum efficiency. This causes an increase in threshold current in the case of a semiconductor laser, and an increase in power consumption and a drop in luminous efficacy in the case of an LED. Furthermore, a rise in injected-carrier density is followed by the screening of the piezo-electric field and a change in emission wavelength.
To solve those problems, using a substrate that has as a surface a non-polar plane, for example, a (10-10) plane which is perpendicular to a [10-10] direction and called an m-plane, is being considered. The sign “−” to the left of a number inside parentheses that indicates a Miller index means a “bar”. The m-plane is a plane parallel to the c-axis (the fundamental vector c) and is orthogonal to the c-plane as illustrated in FIG. 2. Electrical polarization does not occur in a direction perpendicular to the m-plane because, on the m-plane, Ga atoms and nitrogen atoms exist on the same atomic plane. Consequently, forming a layered semiconductor structure in a direction perpendicular to the m-plane does not generate a piezo-electric field in the active layer and thus solves the problems described above.
The term m-plane is a collective name for (10-10) planes, (−1010) planes, (1-100) planes, (−1100) planes, (01-10) planes, and (0-110) planes. Herein, “X-plane growth” means epitaxial growth in a direction perpendicular to an X-plane (X=c, m) of a hexagonal wurtzite structure. The X-plane in an X-plane growth may be referred to as “growing plane”. A semiconductor layer formed by an X-plane growth may be referred to as “X-plane semiconductor layer”.
A gallium nitride-based compound semiconductor light-emitting element is manufactured generally by growing an n-type gallium nitride-based compound semiconductor layer, an active layer, and a p-type gallium nitride-based compound semiconductor layer on a substrate in the stated order. Those gallium nitride-based semiconductor layers are formed by vapor deposition such as metal organic chemical vapor deposition (MOCVD).
Conventionally, a p-type gallium nitride-based semiconductor layer (sometimes referred to simply as “p-type layer” in the following description) is grown by controlling conditions so that magnesium (Mg) which is a p-type dopant is contained at a concentration of 1×1019 cm−3 or higher. Mg atoms contained as dopants in the p-type layer readily bond with hydrogen atoms, and Mg atoms bonded with hydrogen atoms are inert as p-type dopants. Activating heat treatment (annealing) for detaching hydrogen from Mg atoms is sometimes performed for that reason. However, the activating heat treatment activates only a few % of all Mg atoms contained in the p-type layer. In order to obtain a hole concentration of approximately 1×1017 cm−3 which is necessary to exhibit favorable electrical characteristics, the p-type layer therefore needs to be doped with Mg atoms at a concentration 100 times the necessary hole concentration, specifically, 1×1019 cm−3, or more. For favorable electrical characteristics, accomplishing an Mg concentration of approximately 2 to 5×1019 cm−3 is desirable.
Related art concerning p-type doping in a gallium nitride-based compound semiconductor element is disclosed in, for example, Patent Document 1 to Patent Document 3.